Re: [a/s] Pullrite vs Hensley

[owner-airstream@xxxxxxxxxxxxx]

In a message dated 02/19/2000 11:23:25 AM Eastern Standard Time, 
gregoryw@home.com writes:

<<  Hensley is all endorsements.  I still don't understand how it works. >>

Okay...here is more of that newsgroup discussion..it's long and 
technical...but it does explain a bit of how the Hensley works.  Two people's 
posts are included.  The first and the follow up.


The Original Post

I believe that some incorrect or at least over simplified information is
being suggested in this thread. This information is germane to understanding
what the Hensley hitch does or does not do in regards to controlling sway.

Conventional sway between a tow vehicle and travel trailer is not defined as
simple pendulum action by the trailer. When I say conventional sway, I am
referring to the well known fishtailing action associated with sway. There
may, in fact, be another form(less common) which has a characteristic that
more closely represents pure pendulum action. Let me address that later.

Conventional sway is caused by some external force causing the trailer to
yaw about a vertical axis nominally located at the trailer axle. In the case
of tandem axles, approximately midway between the two axles. This yaw action
causes the rear of the trailer to move laterally in one direction and the
front of the trailer to move laterally in the other direction. The front of
the trailer acting through the hitch ball and the approximate 4 to 5 ft
lever arm to the tow vehicles rear axle will cause the tow vehicle to turn
in the same direction as the rear of the trailer. At this point we have the
trailer going one way and the tow vehicle going the other. This is the
characteristic behavior that results in the fishtailing action. The key
point here is that it is not just a simple pendulum action of the trailer
pivoting on the hitch ball. Now on to the Hensley.

Let me qualify my position. I do not own a Hensley, have any financial or
other interest in Hensley. I have, however, studied the Hensley hitch and
believe that I have a good basic understanding of it's operation.

The Hensley hitch, through it's linkage system does in fact lock out the yaw
motion of the trailer(prevent sway). It is accomplished by brute force
through the two links. I built a small model of the hitch linkage system to
illustrate this fact. It does work. This lockout of trailer yaw action
occurs within a range of approximately plus or minus 15 degrees turning
angle between the tow vehicle and trailer. Outside of this range, the
linkages can not lockout the trailer yaw action, due to the
rotation/position of the links.

Now let us consider a possible additional form of sway that acts more like a
simple pendulum, with a pivot point at the hitch ball. I hesitate to say
that this form cannot occur, but it seems to me to be somewhat unlikely. If
the trailer can swing side-to side, like a pendulum, it would have to
literally break traction with the road surface and allow it's wheels to
slide sideways. I guess that this could most easily happen on a slick road
surface(water or ice). Anyway, if this were to happen, the Hensley can not
lockout this motion. Basically this motion is no different that a simple
turn and we know that the Hensley does not prevent the trailer from
returning to an in-line position with respect to the tow vehicle. To be
complete, if this type sway did occur, the pivot point would not be the
hitch ball, but the synthesized pivot point created through the Hensley
linkage system. This synthesized pivot point would be located approximately
at the tow vehicle rear axle, if not further forward.

I have puposely tried to be somewhat brief in this description to keep the
length reasonable.

All criticisms cheerfully accepted

Bob
~~~~~~~~~~~~~ 
The Reply

I agree with your assessment of the physics involved. The difference
with a Hensley is that the pivot point is mechanically transferred to a
point near the rear axle through linkages, very similar to the principal
of a 3 point hitch on a tractor. I believe the inventor got the idea for
the Arrow from his knowledge of farm implements, but I'm not 100% sure.
When towing straight down a highway, the point of convergence (virtual
pivot) is actually out in front of the tow vehicle. As soon as a slight
turn is incurred, this point moves back to near the rear axle by the
simple action of the mechanical linkages. Only when you make a VERY hard
turn (as in backing into a campsite) can you get these linkages to
"breakover" and allow the full 165 degrees of swing. At this point the
pivot is at the hitch, although I can't imagine anyone being able to get
to the "breakover" point doing more than 2 MPH! Actually the principal
of the hitch is very simple, just difficult to explain. There used to be
a URL that explained how this linkage system worked pictorially and it
really clears things up. I use a Hensley and the stability and handling
is truly incredible, and I have also used traditional friction/cam-type
sway control. There is no comparison.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 <<Hunter, since your tow vehicle must out-weigh your trailer, I wonder if the
 hitch is all that critical? >>

Good question. One I can't answer.... The thing for me is... I don't want to 
find out. I don't want to worry about every 18 wheeler coming by... I *know* 
it won't be a problem.  My truck does outweigh my trailer buy over 1,000 
pounds.  I don't want to be in a situation where I have to find out if I can 
get the trailer back in a fishtailing situation.  I do know the trailer can 
sway no matter what the truck weighs.... maybe the truck can handle it. It's 
a great tow vehicle, heavy, powerful, long wheelbase... I just don't want to 
have to rely on my ability to handle a scary situation like fishtailing. 

For me the price of the hitch is a small price to pay for the peace of mind.

It's like buying fire insurance for your home, you pay for it all your life 
and hope you never have to use it.  

Hunter